1. Field of the Invention
The present invention generally relates to a microstrip band pass filter. More specifically, the present invention relates to a microstrip band pass filter using end-coupled stepped impedance resonators (hereinafter, referred to as “SIR”) that can be used in a millimeter wave band.
2. Discussion of Related Art
A microstrip line represents a transmission line that has a pair of conductive thin films at both sides and a dielectric substrate therebetween. It is also referred to as “microstrip transmission line”. The upper conductor has a designated shape (strip type) and the lower conductor is formed of a wide ground conductor. Further, there can exist an up-down symmetrical structure where another dielectric is located on the upper conductor. The SIR is a resonator that transmits a signal in a specific frequency band and blocks signals in other frequency bands based on the impedance ratio and length of the microstrip line.
As a filter using the conventional SIR, a type of using a dielectric coaxial line and a planar type of a printed circuit are mainly used.
Among these, most of the dielectric coaxial line types, which are original filters that use the SIR, generate a capacitance through a metal structure to make a filter using a two-step SIR, which are appropriate for high output power and are typically used in a low frequency range such as below several GHz.
For the planar type of the printed circuit, there generally exist a strip line, a microstrip line and a coplanar waveguide (hereinafter, referred to as “CPW”). Among these, a filter using the strip line and the microstrip line employs a three-step SIR to connect the coupling structure between resonators to the parallel coupling line, which has been mainly reported in a frequency band below 10 GHz. A filter using the CPW employs a two-step SIR to connect the coupling structure between resonators using a gap and a ground, which has been reported in a frequency band below 300 GHz.
Most of the conventional planar type filters using the SIR are coupled to the resonators through the coupling line, which have been used in a frequency band below 10 GHz. Further, since a capacitance of the coupling line is changed together with an even mode impedance and an odd mode impedance according to the gap, it is sensitive to the manufacturing process error and it is difficult to have good attenuation and narrowband characteristics. Therefore, the conventional planar type filters using the SIR are required to have the good attenuation and narrowband characteristics, and as the filter structures becomes smaller proportional to a wavelength, there occurs a problem that it is not suitable for use in the millimeter wave band (30 GHz to 300 GHz) that requires an insensitive characteristic to the small manufacturing process error. Further, contrary to the filter that has been used in the frequency band below 10 GHz, the filter of the millimeter wave band should consider the parasitic component caused by the discontinuous structure.